Subs can sneak attack. That means they can pass an enemy occupied sea zone and attack the next (an enemy destroyer prevents/blocks a sneak attack).

Specific Version 2 rule set

Submarines can submerge after first attacker/defender roll dice. Thus an attacking fighter still has the chance of hitting a sub before it is able to submerge. Enemy destroyers stop subs from submerging.

Specific Version 3 rule set

Sea transports do not defend. Neither can they be selected as casualties. They are just lost if enemy wins the sea zone. For example a bomber can defeat an infinite amount of unguarded transports.

Submarines can submerge before any roll. So an attacking fighter has no chance of hitting a submarine, unless a destroyer prevents it from submerging.

ListenpunktBombers during strategic bombing do not destroy enemy PUs, but do damage structures. Enemy will have to spend PU to repair structures.
Please find detailed information regarding rules & specific unit capabilites in the relevant notes tab inside the game.